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United States Patent |
5,314,939
|
LeBarny
,   et al.
|
May 24, 1994
|
Amorphous copolymers for photorefractive compounds used in optical
signal processing
Abstract
The invention is a series of amorphous copolymers which, used in
association with a small molecule, provide photorefractive materials that
can be used in optical signal processing. These copolymers contain an
active non-linear optical group and a group which contributes to their
photoconductivity.
##STR1##
where: X is H, CH.sub.3, Cl or F
Y is H, CH.sub.3, Cl or F
2.ltoreq.n.ltoreq.4
2.ltoreq.m.ltoreq.4
The electron-donor group D being: --O--, --S--,
##STR2##
or --COO-- The electron-accepter group A.sub.e being: --CN, --NO.sub.2,
##STR3##
Inventors:
|
LeBarny; Pierre (Orsay, FR);
Broussoux; Dominique (Marcoussis, FR);
DuBois; Jean-Claude (St. Remy Les Chevreuses, FR)
|
Assignee:
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Thomson-CSF (Puteaux, FR)
|
Appl. No.:
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937719 |
Filed:
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October 6, 1992 |
Foreign Application Priority Data
Current U.S. Class: |
524/241; 252/582; 524/544; 524/547; 524/548 |
Intern'l Class: |
C08L 039/04 |
Field of Search: |
524/241,259,548
252/582
|
References Cited
U.S. Patent Documents
4867538 | Sep., 1989 | Yoon et al. | 252/299.
|
4898691 | Feb., 1990 | Borzo et al. | 252/589.
|
5185102 | Feb., 1993 | Harelstad et al. | 252/582.
|
Foreign Patent Documents |
0244288 | Nov., 1947 | EP.
| |
0090282 | Oct., 1983 | EP.
| |
0240276 | Oct., 1987 | EP.
| |
0260687 | Mar., 1988 | EP.
| |
Other References
Williams, D. J. (1984) Angew. Chem. Int. Ed. Engl. 23, 690-703, "Organic
Polymeric and Non-Polymeric Materials with large Optical Nonlinearities".
S. Ducharme et al., (1991) Phys. Rev. Lett. 66 (14), 1846-1849 (Apr. 8,
1991) "Observation of the Photo refractive Effect in a Polymer".
D. DC. Bradley et al., (1991) Phys. Rev. Lett. 67 (18), 2589 (Oct. 28 1991)
"Comment on observation . . .".
S. Ducharme et al. (1991) Phys. Rev. Lett. 67 (18) 2590 (Oct. 28,
1991)"Reply to Comment".
R. P. Feynman et al., Feynman Lectures on Physics, vol. 1 33-3-33-6
Addison-Wesley, Reading MA, 1963.
|
Primary Examiner: Schofer; Joseph L.
Assistant Examiner: Nagumo; Mark
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt
Parent Case Text
This is a division, of application Ser. No. 07/734,673, filed on Jul. 23,
1991, now U.S. Pat. No. 5,198,514.
Claims
What is claimed is:
1. A photorefractive mixture of organic compounds consisting of an
amorphous copolymer for photorefractive compounds having the following
chemical formula:
##STR9##
where: X=H, CH.sub.3, Cl or F
Y=H, CH.sub.3, Cl or F
.ltoreq. n.ltoreq.4
2.ltoreq.m.ltoreq.4
D is an electron donor group being --O--, --S--,
##STR10##
where 0.ltoreq.p.ltoreq.3, or --COO-- A.sub.e is an electron-acceptor
group being --CN, --NO.sub.2,
##STR11##
##STR12##
is a system rich in .pi. conjugated electrons:
##STR13##
and a small electron-acceptor molecule C, the molar ratio of electron
donor groups D to small electron-acceptor molecules C being between 0 and
1.
2. The photorefractive mixture described in claim 1 in which the small
electron-acceptor molecule is 2,4,7 trinitro-9-fluorenone.
3. An optical device which uses the photorefractive mixture described in
claim 1 or claim 2.
Description
BACKGROUND OF THE INVENTION
1. Field of the invention
The invention consists of copolymers associated with certain small
molecules which form photorefractive compounds usable in optical signal
processing. More specifically, it concerns new families of amorphous
polymers whose lateral chains contain chromophoric groups capable of
generating non-linear optical effects and electron-donor groups which,
associated with electron-accepter groups, give the material its
photoconductive properties.
A material is said to be "photorefractive" when illumination generates
remanent variations in the refractive index. When an electric potential is
applied to these materials and they are exposed to incident distributed
luminosity, variations in the refractive index occur in the dark,
isolating section while this variation tends towards zero in conductive
illuminated zones where charge migration cancels out the voltage. When the
illumination stops, the material returns to the initial situation except
close to the boundary: the dielectric on the illuminated side of the
boundary is depleted in photon-carriers which have migrated a certain
distance and become trapped in the dark area. At the boundary, the space
charges created form a local field which results in a local variation in
the refractive index. This variation remains throughout the period
required for dielectric relaxation of the material (several months in the
case of compounds such as lithium niobate).
2. Description of the prior art
The photorefractive effect therefore records a spatial variation and is
used in applications such as:
the recording of volumetric holographic networks
experiments in dynamic interferometry
image processing
the deflection of laser beams
optical amplification
auto-resonant cavities
optical calculation (for association memories, elementary logic components,
reconfigurable interconnections).
At present, most photorefractive materials are inorganic compounds such as:
ferroelectric crystals, for example LiNbO.sub.3, BaTiO.sub.3
sillenites such as bismuth and silicon or germanium oxide crystals
(Bi.sub.12 SiO.sub.20 or B.sub.12 GeO.sub.20)
III-V compounds (GaAs doped with Cr, InP doped with Fe).
However, these materials present certain disadvantages which impede further
development: the compounds which offer the best performance are not
available as large monocrystals and they are still very expensive. In
addition, at present, none of these materials offer the qualities
required, which are very fast response, high sensitivity and large memory
capability.
In parallel with the development of these inorganic photorefractive
compounds, organic materials, particularly polymers, have been found to
possess promising properties:
for non-linear optical applications. Since the beginning of the 1980s, work
on amorphous polymers doped with highly hyperpolarizable molecules and
then amorphous copolymers in which the chromophoric molecules are directly
bonded have shown that the electrooptical properties that could be
obtained were comparable with those of inorganic compounds (U.S. Pat. Nos.
4,828,865 and 4,808,332, taken out by HOECHST CELANESE, the applicant's
European patents EP 88 0579 and EP 89 11327).
in photoconductive applications. These polymers can possess conjugated
multiple bonds as in polyacetylenes and trans polyphenylacetylenes. They
can include aromatic groups consisting of several benzene cores, either in
the structure or in the lateral chains, for example derivatives of
anthracene, pyrene or acridine. They can also possess aromatic amine
functions like, for example, polyvinylcarbazole (PVK). PVK doped with 2,
4, 7 trinitro-9-fluorenone (TNF) is commercially available and has
practical applications in the reproduction of documents (Xerography).
These polymer materials are also generally easy to work and can be used to
produce large surfaces at far less cost than inorganic materials.
SUMMARY OF THE INVENTION
Consequently, this invention proposes a series of amorphous copolymers with
properties which are advantageous both in non-linear optics and in
photoconductivity and can be used to produce organic photorefractive
materials for optical applications, particularly optical signal
processing. These amorphous copolymers with lateral chains comprise:
a group with a high .beta. hyperpolarizability value to generate strong
non-linear optical effects, molecular engineering making it possible to
adjust this hyperpolarizability and the transparency of the material to
suit the wavelength used
an electron-donor group which, in conjunction with a small
electron-accepter molecule, makes the copolymer photoconductive.
Preferably, these copolymers comply with the following chemical formula:
##STR4##
where: X is H, CH.sub.3, Cl or F
Y is H, CH.sub.3, Cl or F
2.ltoreq.n.ltoreq.4
2.ltoreq.m.ltoreq.4
The electron-donor group D being: --O--, --S--,
##STR5##
or --COO-- The electron-accepter group A.sub.e being: --CN, --NO.sub.2
##STR6##
representing a system rich in .pi.-conjugated electrons:
##STR7##
This invention is also a photorefractive mixture obtained by introducing a
small electron-accepter molecule into a copolymer complying with the
invention; this molecule may be 2, 4, 7 trinitro-9-fluorenone.
Finally, the invention includes the use of this organic mixture in any
device which uses the photorefractive effect.
The invention will be better understood, and other advantages will become
clear, upon reading the following description, which is not exhaustive,
and by studying the appended figures including:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1, showing a family of copolymers complying with the invention,
FIG. 2, showing a reaction diagram of the synthesis of the colorant monomer
in one example of a copolymer complying with the invention,
FIG. 3, showing a reaction diagram of the synthesis of the monomer with an
electron-donor group in an example of a copolymer complying with the
invention.
DETAILED DESCRIPTION OF THE INVENTION
In general, copolymers complying with the invention are preferably produced
by radical polymerization using azobisisobutyronitrile (AIBN) as a starter
and dimethylformamide as a solvent at 60.degree. C. for twenty four hours.
The copolymer is precipitated in ethanol.
The colorant content is determined by visible UV spectrometry. A precise
example of the synthesis of a copolymer complying with the invention and
its non-linear optical and photoconductive properties are given below:
synthesis of poly [N (methacryloyloxyethyl) carbazole co 4 (N
methacryloyloxypropylamino) nitrobenzene)].
##STR8##
1) The colorant monomer 4'(N-methacryloyloxypropylamino) nitrobenzene is
synthesized from 4 fluoronitrobenzene in 2 stages as represented by the
reaction diagram in FIG. 2. The experimental details are described in
patent 89 11327 deposited by the applicant.
2) The monomer N methacryloyloxyethyl carbazole is synthesized from
carbazole in 2 stages as represented in the reaction diagram in FIG. 3:
1st stage: synthesis of N (2 bromoethyl) carbazole.
74 ml of 50% sodium hydroxide solution is prepared in a 500 ml receptacle.
83 ml of 1,2 dibromoethane (0.96 moles), 10 g of carbazole (0.06 moles)
and 1.72 g of benzyltriethylammonium chloride are added. The mixture is
heated to 70.degree. C. for 10 hours with strong agitation. The contents
are poured from the first receptacle into a second receptacle containing a
mixture of water and hexane. The raw product precipitates and is separated
by filtration. To eliminate any carbazole which has not taken part in the
reaction, the solid obtained is placed in chloroform and agitated. After
filtration through sintered glass, the filtrate recovered is evaporated
then recrystallized in ethanol. The melting point of the small beige
needles obtained is 140.degree. C. The reaction yield is 35%.
2nd stage: synthesis of N (methacryloyloxyethyl) carbazole.
5 g of N bromoethyl carbazole (0.018 moles) is placed in a 50 ml receptacle
and 25 ml of hexamethylphosphortriamide (HMPA) is added. The mixture is
agitated until the N bromoethyl carbazole is completely dissolved. The
next step is to add 3.35 g of lithium methacrylate (0.036 moles) and 0.2 g
of hydroquinone to avoid polymerization. The mixture is then agitated for
24 hours at 40.degree. C. When poured into water, the reaction mixture
gives a white precipitate which is filtered out through sintered glass and
then washed with water and hexane to remove excess lithium methacrylate,
HMPA and hydroquinone.
This gives a white powder which is purified by chromatography using toluene
as the eluant. It is then recrystallized in ethanol. The reaction yield is
45% and the product obtained has a melting point of 84.degree. C.
3) Copolymer synthesis
3.14 mg of AIBN, 1.41 g (5.05 moles) of N methacryloyloxyethyl carbazole,
0.608 g (2.30 moles) of 4 (N methacryloyloxypropylamine) nitrophenyl and
20 ml of dimethylformamide (DMF) are placed in a previously-sealed
ampoule. This reaction medium is frozen in liquid air then placed under
vacuum. It is allowed to return to ambient temperature and the above
operation is repeated twice. Finally, the ampoule is sealed under vacuum
and heated to 60.degree. C. for 24 hours. The copolymer is precipitated in
ethanol. The copolymerization yield is 61.5%. The colorant content is
determined by visible UV spectrometry: x=0.237.
The copolymer vitreous transition temperature is 136.5.degree. C.
When dissolved in 1,1,2 trichloroethane, this copolymer allows thin films
to be produced easily.
The copolymer film is dried under vacuum at 120.degree. C. and oriented by
a steady electric field. This field is created by ionizing the air feed
from a metal tip at very high voltage (4 kV) using the corona method.
Polarized in this way, the polymer doubles the frequency of a YAG laser
operating at 1.06 .mu.m. The d.sub.33 factor measured is 3 pm v.sup.-1
(identical to that obtained with BSO (bismuth silicon oxide)).
This same copolymer, when doped with 20% TNF, gives a film which, after
orientation by the corona method (V=4 kV), doubles the frequency of a YAG
laser (.lambda.=1.06 .mu.m) with d.sub.33 =4 pm V.sup.-1.
This copolymer, doped with 20% TNF, and placed between glass coated with
ITO (a mixture of indium and tin oxides) and a chrome-gold electrode has
photoconductive properties.
In fact, the .beta. sensitivity, defined by the following equation:
.sigma.=.sigma..sub.o +.beta.I
where
.sigma.=conductivity when illuminated
.sigma..sub.o =conductivity in the dark
I=intensity of the luminous flux
is approximately 0.5 10.sup.-10 .OMEGA..sup.-1 cm W.sup.-1 for a 514 nm
incident wavelength. This .beta. coefficient is approximately 10.sup.-9
.OMEGA..sup.-1 cm. W.sup.-1 for polysilanes and approximately 10.sup.-7
.OMEGA..sup.-1 cm. W.sup.-1 for the BSO photorefractive crystal.
These electroactive and photoconductive copolymers can be optimized by
carefully selecting the chromophoric and photoconductive groups, their
relative proportions and, above all, by using these materials at the
wavelength for which they were optimized.
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